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BRAIN GENDER

THE EFFECTS OF SEX HORMONES ON THE BRAIN
Certain regions of the brain are involved in sex drives and sex specific behavior; lesions in these regions can affect sex drives and behavior. Hormones bind to many target tissues and cause changes there. Estrogens and androgens also have target cells in the brain; the primary targets are the same regions that are identified as controlling sex behavior. The sex hormones in animals not only influence many brain regions involved in sexual behaviors, they also cause the sexually dimorphic sizes in some of these same brain regions. All available evidence suggests the same in humans. For example, the hypothalamus and amygdala equivalent between males and females at birth but sexual dimorphism exists in adults (Darlington, 2002).

BRAIN DIMORPHISM
There are a number of sexually dimorphic brain nuclei such as regions of the preoptic area, bed nucleus of the stria terminalis, and suprachiasmatic nucleus. (Kruijver, 2001). The anterior commissure is larger in females (Darlington, 2002). Onuf's nucleus of the spinal cord is sexually dimorphic in humans (Hines, 2004). Studies have reported that the corpus callosum is larger in females, although there is some dispute over this conclusion (Bishop, 1997). One study found that the caudate and amygdala were relatively larger in female brains and the globus pallidus and hippocampus were relatively larger in males (Giedd, 1997). Higher numbers of neurons exist in male brains while higher numbers of neural processes exist in female brains (Rabinowicz, 1999). Women have a larger percentage of gray matter to white matter than men although the overall size of the brain is greater in men (Cosgrove, 2007). In mice and humans, the sexual dimorphism in the INAH-1 (Interstitial nucleus of the anterior hypothalamus) begins after age four. The gender differences in the SCN (which has twice the size and twice the cell number in males) disappears with age (Sickel from Steiner, 2000). The massa intermedia is present in only 78% females and 68% males (Sickel from Steiner, 2000).

Brain function seems to vary between genders, especially with regard to brain lateralization. In reading tasks, women use their both their hemispheres while men more exclusively use their left hemisphere. Women's brains seem to be less lateralized in that damage to one area is less likely to be debilitating-there are other brain regions which perform the same function or can compensate. Women are more likely to use both halves of their brain to a greater degree in specific tasks than men (Pugh, 1996; Moir, 1991). Female brains possess a higher percentage of gray matter and much of the gender difference in brain size is due to a greater amount of white matter in male brains (Gur, 1999).


Overall blood flow to the brain is higher in women during both resting and cognitive conditions. In women, brain glucose usage is higher than men (Cosgrove, 2007).
Gender differences exist in the dopamine, serotonin, and GABA circuits. For example, men synthesize serotonin faster than women but blood levels of serotonin are higher in women than in men. Gender differences in serotonin reuptake have been observed which may be relevant to the gender difference in rates of depression. Women have higher amounts of dopamine and availability of dopamine transporters than men in specific brain regions which may explain the greater vulnerability to schizophrenia in men. GABA levels vary throughout the menstrual cycle (Cosgrove, 2007). Some studies have identified gender differences in the perception of vision, sound, and some odorants (Darlington, 2002). Gender differences in EEG readings and differences throughout the menstrual cycle are known and visual acuity varies over menstrual cycle (Darlington, 2002). Androgens influence a number of non-sexual mental abilities in men. Normal spatial abilities require a certain level of circulating androgens (Gooren, 2002). Female brains produce a surge of LH induced by the hypothalamus after estrogen exposure. Male brains and female brains exposed to testosterone do not respond this way (Sickel from Steiner, 2000).

A number of other gender differences in the nervous/endocrine systems have been reported, including gender difference in the lateralization of brain areas used in the perception of an illusion (Rasmjou, 1999), the startle reflex (Swerdlow, 1999), leptin concentrations (higher in newborn females than males; Maffeis, 1999), brainstem response to auditory stimuli (Lopez-Escamez, 1993), the response of the opioid system in several regions of the brain (Zubieta, 1999), metabolic differences in certain areas of the brain (Sijens, 1999), in Sylvian fissure morphology (Foundas, 1999), spatial memory tests (Potsma, 1999), the area of the brain used in odor discrimination and performance in odor discrimination tests (both higher in females; Yousem, 1999), the regions of the brain which degenerate with age (Oguro, 1998), mouth shape during speech (Hausmann, 1998), lateral symmetry in auditory processing in newborns (Sininger, 1998), schizophrenia incidence and age of onset (Salem, 1998), brain activity regions in patients with temporal lobe epilepsy (Savic, 1998), brain lateralization in language tasks (Jaeger, 1998), recovery time after brain injury (Groswasser, 1998), and brain torque in schizophrenic patients (Guerguerian, 1998).

 

Although brain dimorphism is often interpreted as occurring early in life, it is possible that later hormonal actions cause this dimorphism. In rodents, the levels of steroid hormones in adults can change the size of brain nuclei (such as the medial amygdala) and this should be considered as a possible confounding effect on other brain studies (Cooke, 1999). Although both men and women suffer a decrease in brain volume as they age, the frontal and temporal lobes are more affected in men while the parietal lobes and hippocampus are more affected in women (Cosgrove, 2007).

 

BRAIN DISORDERS

Gender differences have been documented for a number of mental and behavioral disorders. Men more frequently suffer from retardation, language disorders, stuttering, autism, attention deficit disorder, Tourette's syndrome, substance abuse, sexual and gender identity disorders, schizophrenia, and antisocial disorders. Women more frequently suffer from depression, dysthymia, seasonal affective disorder, panic, generalized anxiety disorder, social phobia, multiple personality, eating disorders, and thyroid disorders. The only form of bipolar disorder which is more common in women is rapid cycling bipolar disorder (in which there are four or more episodes a year) (Halbreich from Steiner, 2000; Rasgon from Steiner, 2000; Lippa, 2002; Darlington, 2002). Menstrual changes increase epilepsy (Darlington, 2002). The gender differences in depression rates begin in the teen years (Rasgon from Steiner, 2000).

Autistic individuals possess some level of awkwardness in social settings, have some abnormality in their communication skills, and typically display some form of obsessional behavior. While some possess high intelligence, others possess learning problems. Some may have little or no language. In one model, autism is the extreme expression of male characteristics of the brain-skill in systemizing and less in empathizing (Baron-Cohen, 2003).

Turner syndrome women are more likely to be affected by disorders which are more common in males, such as autism, attention deficit, and perhaps schizophrenia. Brain differences between Turner syndrome and normal women suggest that some of the pseudoautosomal genes on the X chromosome have roles in brain development (Davies, 2006).


Men suffer more from cardiovascular disease and diseases caused by microbes while women make up the majority of patients of autoimmune disorders, fibromyalgia, and chronic pain (Kajantie, 2006). Stress responses (both of the autonomic nervous system and of the hypothalamic-pituitary-adrenal axis) are lower in women than in men in general. These responses in women vary with age (relative to puberty and menopause) and phase of the menstrual cycle (Kajantie, 2006). Women suffer more from rheumatological disorders